Сетевая библиотекаСетевая библиотека
Kingdom of Plants: A Journey Through Their Evolution Will Benson This book accompanies the landmark television series Kingdom of Plants 3D: With David Attenborough.In the last 500 million years, plants have undertaken an epic journey that has not only spanned the ages but has altered the very make-up of the planet. It was a journey that began in a dark and barren world, and has culminated in a planet that is draped in rich colours, and overflowing with a diversity of mystifying orchids, exploding seed pods and snapping carnivorous plants.But plants are far more than just beautiful and bizarre. Through the chapters of this book we uncover how plants first began to live on land, how they have become linked with a multitude of animal and fungi partners, and how ultimately they have shaped both landscapes and cultures.With an introduction by Professor Stephen Hopper, Director of the Royal Botanic Gardens at Kew, as well as contributions from leading botanists and horticulturists, this book unpicks the strands of our planet’s network of botanical life. Through its pages we reveal the extraordinary ways in which plants have come to live and thrive in all habitats, and we discover how they can provide us with answers to many of the problems that face humanity in our modern age.For more information, please visit:www.kingdomofplants.com Kingdom of Plants A Journey Through Their Evolution Will Benson FOREWORD BY Professor Stephen Hopper DIRECTOR, ROYAL BOTANIC GARDENS, KEW Contents Title Page (#ud9a2a110-4f6c-56d3-981c-c916b27d30ef) Foreword by Professor Stephen D. Hopper (#ulink_5635ae67-f3b3-5cd1-858e-f17b53018b62) Kingdom of Plants 3D with David Attenborough (#ulink_f42d3ffe-0038-5ca4-989f-6ee4198e2c38) Chapter One: The Evolution of Plants (#ulink_a8727ccc-841d-5abc-a38a-8790fbac0b0d) Chapter Two: The Wonder of Flowers (#ulink_84b5679f-8569-5cf1-942b-231be37c7eaa) Chapter Three: Form and Function of Plants (#ulink_096e1879-47f3-5c9a-abeb-3cbbd08c14be) Chapter Four: How Plants Have Shaped the World (#litres_trial_promo) Chapter Five: Plant Communication (#litres_trial_promo) Chapter Six: Plants and Animals (#litres_trial_promo) Chapter Seven: Extreme Survival (#litres_trial_promo) Chapter Eight: The Power of Fungi (#litres_trial_promo) Chapter Nine: No Plants, No Humans (#litres_trial_promo) Acknowledgements (#litres_trial_promo) Searchable Terms (#litres_trial_promo) Copyright (#litres_trial_promo) About the Publisher Foreword by Professor Stephen D. Hopper (#ulink_fa4efda1-d1be-51a4-8cc4-4bea540159db) Plant diversity underpins most life on Earth, including ours. Plants provide the oxygen for every breath we take. Think of our daily ingestion of water and food, staying healthy through medicines, living comfortably in buildings, the joys of music, reading, gardening, farming, growing trees and exploring nature. This book reveals some of the wonders of the plant world, exploring scientific information on the evolution of plants, the wonder of flowering plants, their diverse form and function, and their vital place on Earth. Stories abound of plants having a profound impact on our planet throughout evolutionary history, right up to the present day of unprecedented global change. It is thrilling for me to revisit this great journey in these pages, from the remarkable evolution of photosynthesis in primeval oceans to the great move onto land through the evolution of desiccation-tolerant plant bodies, pollen and seeds. Who could not marvel at the diversity of the world’s estimated 400,000 flowering plants? Few are not moved by the sheer beauty and intellectual challenge of understanding how such rampant life came to be. This book eloquently tells that story in an accessible up-to-date text. For those whose interest is more excited by practical uses or the conduct of human affairs, there is much food for thought here as well. Apart from providing us with oxygen and moderating our climate, we learn that plants form a rich foundation for the web of life, and touch the lives of all people in every nation. A special appeal of this book is its celebration of plant scientists and the insights they continue to bring. Charles Darwin devoted the best part of his last 20 years to the experimental study of plants, realising their value in illustrating evolution by natural selection, and publishing more books and journal pages on plants than on his more celebrated works in geology and zoology. Today, there are more plant scientists alive than ever before, and their discoveries in the field and laboratory are just as compelling. Much remains to be done in the exploration of plant diversity. We are still discovering and naming 2000 new species of plant on Earth each year, from rainforest trees to colourful shrubs and orchids of temperate climates (especially in the southern hemisphere). Plant diversity unquestionably underpins human existence and livelihoods, yet we continue to destroy it at an alarming rate, with one in five plant species recently estimated to be under threat of extinction. In our rapidly changing world, we are at a turning point for plant diversity. Without a fundamental shift towards more active conservation and the sustainable use of plants, our prospects for the future are grim indeed. Communicating this message in exciting and innovative ways to mass audiences remains essential. I am delighted that the Royal Botanic Gardens, Kew, has collaborated recently with Sir David Attenborough and Atlantic Productions in providing the plants and location for the astonishing series Kingdom of Plants 3D. I commend this programme and this beautifully produced book to all those who share a sense of wonder in plants and an appreciation of their central place in the lives of all on our one breathing planet. Professor Stephen D. Hopper Director (CEO & Chief Scientist) Royal Botanic Gardens, Kew April 2012 Kingdom of Plants 3D with David Attenborough (#ulink_87359962-9ea0-5670-9ed7-94706d04d0d9) ‘Watching flowers develop and insects visiting flowers is always wonderful to see, but in 3D it becomes transcendental. Seeing this happen in such depth gives you a most extraordinary, vivid impression. It’s unforgettable.’ - David Attenborough To preview the Kingdom of Plants app, scan the code above, and embark on your own journey with David Attenborough as he uncovers the hidden realm of plants at Kew. This book, which explores both the splendour and diversity of the world of plants, was written to accompany the landmark television series Kingdom of Plants 3D with David Attenborough, produced by Atlantic Productions for Sky3D. Using cutting-edge stereoscopic cinematography, this revolutionary 3D project follows David Attenborough in his 60th year of broadcasting, as he uncovers the most amazing spectacles of the botanical world. Filmed over one year at the Royal Botanic Gardens, Kew, this series captures time-lapse footage of flowers as they burst into bloom, captivating interactions between plants and insects in microscopic detail, as well as snapping carnivorous plants and mystifying orchids – all filmed for the first time in three dimensions. Discover how plants first began to live on land, and how they have come to fill their place in the natural world today. (#ulink_48e8fe10-ad1b-5d52-be63-d46dc4222f38) © RBG Kew ‘With the spread of plants onto land, their shapes and sizes soon began to diversify.’ In the last five hundred million years, plants have undertaken an epic evolutionary journey that has altered the very make-up of the planet. This journey began in a dark and acidic world and continues today in a world of rich colours, ornate shapes and mesmerising smells. Every single step of their incredible evolutionary journey has been integral to creating the world we live in today. It is only by pulling apart the threads which create this rich network of flora that we can begin to uncover the extraordinary ways in which plants have come to live and thrive in all environments. From their humble beginnings plants have become progressively complex and increasingly important to life on Earth. It is only through the success of the plant life on our planet that the animals that walk the Earth can be supported and it is the success of plants that has allowed humans to exist. Ultimately it is plants that will secure a place for our species to exist long into the future. Blue morpho (Morpho peleides) Plants today help support vast ecosystems of interconnected species. © RBG Kew Over millions of years plants have become increasingly defined and specialised, carving out their own niches on the surface of the planet; each one striving for the evolutionary equivalent of the limelight – a chance to reproduce and spread their genes. In the beginning of the journey we uncover the origins of photosynthesis, an elegant mechanism which revolutionised the way organisms could obtain energy, providing a powerhouse through which plants could grow and compete. In the next step, plants make a crucial leap out of their watery beginnings, as they moved onto the land. But it is from here that the botanical world really came into its own. Plants became tough and tall, developing structural wood that allowed them to reach new heights. They developed mighty anchoring roots and broad, air-pumping canopies. Skipping forward a few hundred million years we meet the rise of the first flowers, signalling the beginning of a great and ground-shaking love affair between insects and plants, setting in motion a course of events that would bind some plants to the animal world. From this point, the pace of the story picks up as we enter the age of rich biodiversity and intricate connections between Earth’s flora and fauna; we discover the extraordinary relationships between plants and animals, and we see that from one form of simple plant, many millions of diverse species have now evolved. And yet the most crucial chapters in the story of plants are still being written From the early 1800s onwards countless expeditions set off to chart new territory and to collect rare and beautiful botanical specimens with which to fill elaborate glasshouses. This was to be the golden age of plant hunting, and with it began humankind’s fascination with the plant world. Today the spirit of these first pioneers remains in the world’s premier botanical gardens and research institutes. The biologists and ecologists who populate these centres have provided us with a far greater understanding of our planet’s biodiversity than ever before. Ultimately it is these botanists and conservationists who will ensure the survival of the world’s plant species. * * * Magnolia sp. One of the many species that were brought to Europe and are now common. © Will Benson To discover the origins of the plant kingdom we have to visit the Earth three billion years ago. At this time a dark gas-filled sky looms ominously overhead, the air is thick and acrid from the smouldering funnels of volcanic activity, and the waters of warm, shallow tropical seas lap at the shores of recently formed magma islands. This is a time we know now as the Archaean eon. In the watery depths of these ancient oceans tiny single-celled organisms drifted through the murky sediments of the seas. These basic microscopic cells were early bacteria, and consisted of nothing more than a simple outer membrane containing just a few primitive proteins inside. Over time, these cells grouped together to form layers of slime across the ancient seabed. Stromatolites These fossils in Australia’s Shark Bay represent the earliest known life forms on Earth. © Minden Pictures/SuperStock These bacteria survived by absorbing near-infrared light from the sun’s rays that penetrated the ancient atmosphere. This light was used to convert the carbon dioxide and hydrogen-based organic chemicals that they had ingested from the water into sulphates or sulphur, providing them with nutrients. Although this basic chemical conversion may seem simple and insignificant, it was in fact the origin of all plant life we see on our planet today. This chemical conversion is the mechanism which all species of plant and animal on Earth, either directly or indirectly, now use as their ultimate source of energy. This was photosynthesis – the use of the light energy to manufacture vital organic food. The first photosynthetic bacteria had bundles of light-absorbing pigments enclosed within their cell walls. These pigments were called bacteriochlorophylls, the predecessors of chlorophyll. The ability for these early cells to use energy from the sun to create organic compounds and sugars which could then be used for growth and movement was a major step forward in evolutionary terms. No longer would these Archaean bacteria be confined to absorbing the mere chemical scraps of nutrients available in the sediment. With their gradual radiation throughout the Archaean seas, the bacteria developed and adapted, and over hundreds of millions of years they evolved significantly. Then, around 2.7 billion years ago, there emerged a further advance in these organisms’ energy-exploiting capabilities. Alongside the early bacteria, new cells appeared – the cyanobacteria. Now while the early bacteria were making use of the ‘invisible’ near-infrared light from the sun, the structure of the pigments in the cyanobacteria’s light-absorbing machinery had evolved to absorb visible light as a means of breaking down chemical compounds to produce food. To help them absorb this visible light even more efficiently they developed a far more varied range of photosynthetic pigments, called phycobilins and carotenoids, as well as several forms of what we know today as chlorophyll. With the change in the wavelengths of light that these new pigments were able to absorb came a change in the precise chemicals that they could digest. For over 300 million years since the rise of photosynthetic bacteria, the by-product of photosynthesis had been sulphurous gases, but in the case of cyanobacteria the by-product was a simple yet vital molecule – oxygen. So successful were the oxygen-pumping cyanobacteria in the prehistoric world that great colonies of them, many billions of cells strong, are now found fossilised in the layers of sediment which were laid down during the Archaean and Proterozoic eons. This record of life, captured as it was over two billion years ago, marks a critical juncture in Earth’s history. As the cyanobacteria went about their business absorbing carbon dioxide from the sea and churning out oxygen into the water, some of this oxygen began to make its way up into the atmosphere, where it accumulated in great clouds, many thousands of tonnes in weight. At the same time – and for reasons still not conclusively known – others gases such as hydrogen began to decrease in the atmosphere. Crucially, this reduction in atmospheric hydrogen allowed oxygen to start accumulating. Liverworts These may be the closest living relatives to the first plants to live on land. © Tim Shepherd Although they are not technically plants themselves, this so-called Great Oxidation Event earns cyanobacteria their place on the plant wall of fame. Oxygen is the third most abundant element in the entire universe, but until the intervention of these simple bacterial cells, Earth’s oxygen atoms were predominantly locked up in chemical relationships with other elements. Elemental oxygen is so chemically reactive that whenever it gets the chance it will bond to nearly any other available molecule, and in the Archaean and Proterozoic eons, there was certainly no shortage of available hydrogen, sulphur and carbon which it could bind to, creating water (H2O), sulphur dioxide (SO2) and carbon dioxide (CO2). But, critically, what cyanobacterial photosynthesis did was to split single oxygen atoms away from water molecules by using energy from sunlight, and then join two lone oxygen atoms together. By forming a partnership with an identical atom, these pairs of oxygen (or O2, as you’ll know them) had alleviated their want to bond to other chemicals through partnering up with one of their own. For the first time oxygen existed in a stable state, and this meant it could begin to increase in the atmosphere. From the point, two billion years ago, at which cyanobacteria first began to produce significant amounts of oxygen by photosynthesis, it still took a further billion years for the levels of oxygen to reach even half of what they are today. Around 1.6 billion years ago, during the mid-Proterozoic eon, oxygen had risen to comprise about 10 per cent of the Earth’s atmosphere, and by now the cyanobacteria had been joined by a host of varied photosynthesising life. These were the red algae, brown algae and green algae, and for the next 500 million years the soft and slimy, filamentous bodies of these organisms thrived in the oceans of the prehistoric world. As these algae evolved, they increased in complexity, developing advantageous new adaptations. Some developed multiple and specialised cells, allowing them to absorb nutrients and sunlight more effectively, and by dividing labour to different cells their growth and reproduction became more streamlined. Coupled with this, the various different algae packaged their genetic material into a single central nucleus, which distinctly separated them from earlier photosynthetic life. Unlike the cyanobacteria before them, these algae were among the first eukaryotic life forms, made of the type of complex cells that make up all higher life on Earth today. Most importantly, their light-absorbing chlorophyll pigments became stacked and enclosed within a double cell membrane, creating the self-contained photosynthetic structures we know from plants on Earth today. These are called plastids, or when in green algae and plants, chloroplasts. These relatively basic but increasingly ingenious organisms, although still confined to their aquatic habitat, first began to embody what we now recognise as the first plant life on Earth. The vast colonies of red algae (whose colours actually ranged from green to red to purple to greenish black) stretched across the ocean floor, where they absorbed the shorter wavelengths of light that penetrated the murky depths. Brown algae soon became adapted to rocky coasts, attaching themselves to submerged rocks by structures called holdfasts. Crucially, green algae acquired an advantage that enabled them to thrive in the shallow waters of land formations. Unlike most other algae, green algae (known as Charophyceae) are able to survive, and even flourish, in the strong light of exposed shallows. There are very few fossilised remains of the marine algae of this time, as their bodies were soft and easily broken down when they died. The exact stages that they underwent as they moved closer to land are unknown. However, we can deduce that around 500 million years ago green algae from the marine habitats and freshwater lakes washed ashore and became stranded on land. Some of these would have given rise to the first land plants. The earliest trace of plants on land that is recorded in the geological record is of the reproductive spores of a plant from the Ordovician period, some 470 million years ago. Analysis of these spores has revealed tiny structures which resemble those seen in a type of modern-day primitive plant called a liverwort. The Palaeozoic era, which literally means the age of ‘ancient life’, stretched from 543 million years ago to 251 million years ago. From this time onwards scientists have been able to trace individual groups of early life. Around 500 million years ago, in a period within the Palaeozoic called the Cambrian, oxygen levels in the oceans dropped drastically, causing a condition called anoxia, which soon spread across the planet. This may have been the trigger for algae to move from water to land. Many of the free-floating, bottom-dwelling organisms in the sea were killed, and this locked away thousands of tonnes of organic matter in their decaying bodies. As a result, the photosynthetic plankton increased in numbers to exploit the space and the nutrients, and they began to pull great quantities of carbon dioxide out of the atmosphere, in turn releasing large amounts of oxygen. Over a period of a few million years oxygen levels rose from around 10 to 18 per cent, up to as high as 28 per cent. This level has since fluctuated over subsequent geological history, resting today at about 21 percent. So successful were the photosynthetic plankton that they still fill all corners of our oceans today. Just a single drop of water from the top 100 metres of the oceans will contain many thousands of these free-floating organisms. They are still considered to be some of the most important producers of organic matter on Earth. Marchantia polymorpha With no internal vascular system, liverworts rely on a moist external habitat. © Tim Shepherd Cyanobacteria These ancient microbes were the first to produce oxygen by photosynthesis. © Science Photo Library/SuperStock Bryophyte spore capsules Even on land these plants require a partially wet environment for reproduction. © Travel Library Limited/SuperStock Alongside the oxygen released from the oceans, the first land-based plants further increased the amount of oxygen that accumulated in the atmosphere. When the concentration eventually tipped over the crucial 13 per cent mark, the first wildfires became possible, and sparks caused by rock-slides and lightning set huge areas of the ancient landscape alight. Fossilised bands of these charcoaled plants, 430 million years old, have been found today. With an abundance of oxygen now readily available out of the water, and with competition for space and resources under the water increasing, life on land became a more favourable option. But while their soft, moist bodies were well suited to an aquatic life, the warm, dry air would cause their thin cell walls to quickly desiccate. More so, water was still necessary for their reproduction, in order to combine their male and female gametes. Over a period of many hundreds of thousands of years, mutations occurred in the cells of some algae which gave them a chance to live further away from the safety of the aquatic environment. A waxy cuticle developed by some algae helped them resist desiccation, and gradually a layer of cells evolved to form a capsule around the embryo to protect it from exposure to the dry air. In time these desiccation-proofed algae reproduced, giving rise to plants better prepared to live out of water. While large blooms of green algae remained water-bound in lakes and oceans, those which had evolved to live for periods outside the water soon began to lose resemblance to their algal ancestors. The bryophytes, as they are now known, became the first land plants on the planet. Even with their adaptations to terrestrial life, these small green hair-like bryophytes, which we now divide into the mosses, hornworts and liverworts, were still reliant on water, in the form of moisture from swamps and bogs, or dew. As they had only recently left their aquatic environment, in evolutionary terms at least, the bryophytes lacked the ability to carry water and nutrients from the soil to their upper parts, and therefore relied on their bodies to be covered in moisture. Once inside their cells the water had to then pass from cell to cell by the slow process of diffusion. As a result of this, even after 450 million years on Earth, bryophytes have remained small and inconspicuous, confined to the dark, damp habitats. Whereas the lives of their marine ancestors were largely commanded by the ocean currents, the first bryophytes developed primitive root-like structures, allowing them to be anchored to the soil. However, not only were the bryophytes key to all land plants we see today, they are now the third most diverse group of plants, numbering well over 10,000 species on Earth. These plants are closely coupled with many important biological and geological processes, including nutrient cycling in tropical rainforests, as well as playing a crucial role in insulating the arctic permafrost. If the first major step in the story of plants was the development of photosynthesis, and the second was their establishment on land, then the third crucial stage was the development of their ability to grow from their limp origins, to become tall and tough, and to gain reproductive success over rivals. But from the origins of the early bryophytes some 450 million years ago, plants had to overcome two major obstacles, in order for them to diversify into the shapes and forms that we see today – how to get water and nutrients to all those parts that are not in contact with the soil, and how to support these parts without the buoyancy of water. The solution was found in the tracheophytes, or vascular plants as they are more commonly known. Silurian landscape The Silurian period, 444 to 416 million years ago, saw the evolution of the first vascular plants. © dieKleinert/SuperStock The evolution of a vascular system was crucial to the plant world. Vascular plants have been the basis of all terrestrial ecology since their arrival on land. Among the first were a group of branching, 10-centimetre-tall plants called Cooksonia, which have been found in the layers of sediment that were laid down during the Silurian period, 444 to 416 million years ago, and are found most commonly today in the fossil fields of the Welsh Borders. Cooksonia had a simple structure, with no leaves or roots, but an internal system of tubes allowing them to move water from beneath the ground up to their photosynthetic structures, and to evenly distribute fuel throughout their branching arms. This hollow internal channel was created by open-ended cells along the length of their stems. In some cells photosynthetic ability was traded in order to take on a purely structural role in the plant. These cells lost their nucleus and their life-giving organs, and instead their walls became thickened with structural sugars, such as cellulose and a tough material called lignin. These vascular plants began to fortify their walls with woody lignin, which gives plants the structure and strength to sprout upwards, unsupported except by their own woody tissues, a key characteristic that separated them from aquatic plants. Over the next 350 million years, the vascular plants would eventually give rise to cycads, ginkgos, ferns, conifers, and ultimately all flowering plants. By the time vascular plants began to make their mark on land, the story of the origin of plants had already spanned a vast timescale of over 2.5 billion years. Over this period the world had shifted during its fiery volcanic youth, deep in the Archaean eon, its skies became filled with life-giving gases during the Great Oxygenation of the Proterozoic eon, and it had played host to the first endeavours by plants to colonise the land in the Devonian period. By now the world was warm and humid and its surface was dominated by the ancient landmasses of Gondwana and Laurasia. The seas continued to support an increasing array of marine animal life, dominated by filter-feeding bryozoa and a diversity of prehistoric fish, and for the first time animals began to follow in the tracks of plants, and make their way out of the lakes and oceans and onto the land. With the spread of plants onto land, their shapes and sizes soon began to diversify. By possessing tough lignin-enforced stems that allowed them to counter the force of gravity, vascular plants soon evolved into an array of new and fascinating forms. From the vegetation of the early Devonian, which consisted of small plants no more than a metre high, plants soon began to use their rigid stems to reach new heights. Fossils which have been dated between 407 and 397 million years old show evidence of plants which produced thickened body parts completely separate from their water-carrying internal tubes. These additional structures were the first examples of plants producing bark. As well as the emergence of woody body parts like bark, fossils from the Devonian reveal a whole host of novel structures that emerged at that time, giving this period its name of the ‘Devonian Explosion’. Fossils from this time include plants such as Archaeopteris, which had frond-like leaves, and plants like Drepanophycus, which had metre-long roots that could reach nutrients deep in the soil. These first tree-like plants grew in vast numbers alongside rivers and estuaries and began to give height to the first primitive forests, some growing up to 20 metres tall. Other woody plants from the Devonian include Rhacophyton, which is suggested to be the precursor of the ferns, an 8-metre tree with a large crown called Eospermatopteris, and a plant called Moresnetia which is thought to have been the forerunner to seed plants. The Devonian Explosion also gave rise to many familiar plant species. The 12,000 or so species of ferns that still thrive throughout our Earth’s tropical and temperate zones today bear testament to their early success in the Devonian. Ginkgo tree These plants are living fossils, dating back to the Permian period, some 270 million years ago. © RBG Kew Animal life was also taking new shape in the warm Devonian climate. Along the damp forest floor millipedes scuttled through the organic mulch, and the first predatory animals, such as trigonotarbids, thought to be relatives of modern spiders, crawled through the undergrowth searching for a meal. In the seas great armoured fish equipped with powerful slicing jaws were quickly increasing in a variety of shapes and sizes, as they evolved to fill the expanding niches of the marine world at the time. In the Late Devonian, around 360 million years ago, the first of these fish made tracks onto land, giving rise to four-legged, air-breathing amphibians, such as Hynerpeton. The Devonian flora had a fundamental impact on the very nature and substance of the land itself. As the tough, tall forest trees put down their networks of anchoring roots, they began to transform the hard and rocky substrate beneath them into hospitable and nutrient-rich soil. Prior to the plant development of the Devonian, the land surface of the Late Silurian was largely exposed bedrock, near-impenetrable to early root systems. The spread of the land plants of the Early Devonian aided the chemical weathering of rocks, helping break them down and release their mineral nutrients. The plants supplied organic acids from the fungi which colonised their roots, and together with acids given off by the decomposition of plant matter on the top of the substrate, these leached into the rock. This leaching softened the rock, enabling the roots to penetrate further into it, gradually breaking it up into smaller sediments. Over time, as organic matter from the surface was drawn deeper down into the ground, the soil depth progressively increased, allowing it to accommodate longer roots below the ground, and in turn larger trees above ground. As the soils of the Late Devonian and Early Carboniferous progressively deepened and became more developed, plant growth was greatly enhanced. Archaeopteris Archaeopteris was one of the first tree-like plants to appear in the Late Devonian. © Parc national de Miguasha/Steve Deschenes A world of colour With the evolution of flowers, the face of the Earth was transformed forever. © Rob Hollingworth All of the early plants up until the Middle Devonian possessed male and female gametes which required water, in some form, for their fertilisation. As the original aquatic plants had a totally submerged existence, their sperm cells could freely move through the water to fertilise their ovum cells. This method of fertilisation put some obvious limitations on where they could survive, and out of water their reproductive strategy would have been impossible. Although bryophytes lived on land, they still relied on a partially wet environment to transfer their sperm cells and spores to their female gametes. We know from bryophytes living today, such as mosses, that when their surroundings are saturated some species store up several times their own weight in water as a reserve, and they are also able to stop their metabolism if their habitat dries out for long periods. These water-dependent land plants were therefore best suited to colonise the damp tidal shores of lowland streams of the Devonian forests, and mosses and ferns can still be found to thrive in these environments today. The need of these amphibious plants to be linked to a moist external environment for their reproduction would have been very limiting in all but the dampest of habitats, and so any plant that was able to break this reliance on water would have had an immense advantage. In the drier terrain further from the shoreline there would have been an abundance of space, light and nutrients. Natural selection soon favoured plants with the ability to grow and reproduce in the dry air of these new habitats. Their trick to surviving in dry air was to package up their reproductive cells in desiccation-proof capsules that could carry them through the air. Capsules we know today as pollen. Equisetopsida For over 100 million years, horsetails dominated the understorey of the Devonian, Carboniferous and Permian forests, growing up to 30 metres high. © age fotostock/SuperStock The first pollen structures that evolved were tiny packages of genetic material, light enough to be carried on the wind to the female cells of a neighbouring plant. On reaching their destination they put out a little tunnel through which their sperm cells could swim down to achieve fertilisation. For the first time, male and female plant structures were able to swap their genetic information over large distances in the dry air. To maximise their dispersing ability many pollen-bearing plants grew taller, and in time the skies filled with airborne DNA from a multitude of pollen-spewing Devonian flora. Although plants would still require water for photosynthesis, it was now possible for them to colonise new, drier regions of the land. From the coastal forests, plants began to push further into the empty expanses of the ancient world. As pollen plants began to spread their domain further inland, and it became necessary for their gametes to travel over even greater distances to achieve pollination, a further major shake-up occurred in the way in which plants reproduced. This was one of the most dramatic innovations in the evolution of plants on land – the evolution of the seed. The earliest plants which exhibited seed-like structures are known as the progymnosperms, dating back to around 385 million years ago. They included trees like Protopteridium, and the leafy, 10- metre-tall Archaeopteris. The fossils of the trees from this period indicate that some, but not all, possessed structures resembling primitive seeds, suggesting that this was a time when the future of the seed hung in the balance. Like all plants before them, progymnosperms produced spores, but uniquely they were able to produce two separate types – micro-spores and mega-spores. This trait, called heterospory, suggests that progymnosperms were the most likely antecedents of all seed plants. Their ability to create variable spores is thought to have been the crucial intermediate evolutionary stage between plants with free-floating single spores and those with true seeds containing a spore-borne embryo. The first true seed plants, which descended from the progymnosperms over 350 million years ago, were a group of tree-like ferns called pteridosperms, belonging to the major division of plants called gymnosperms. The word gymnosperm literally means ‘naked seed’, as they produce seeds which are not fully enclosed in an ovary. In earlier seed-less plants, the gametophytes were released outside the parent plant, but in the pteridosperms the gametophytes were microscopic in size and retained inside the reproductive parts of the plant. This created a moist ovule in which fertilisation could take place, in essence creating a plant within the parent plant. Coupled with this, these embryonic packages were encased with some starting-off food, meaning that they could be transported, ready to germinate as soon as they found themselves in the right conditions. The protective packaging of these seeds also enabled them to remain dormant after dispersal, and wait until conditions were perfect to grow. This prevented the precious genetic material contained within from being wasted in times of flooding or drought. Today seed-bearing plants are the most diverse group of all vascular plants. The evolution of the seed enabled the proliferation of land plants on the wind, in the water, along the ground and in the stomachs of animals. During the Carboniferous and Permian periods, the gymnosperms evolved prolifically, with their extant relatives today including conifers such as pine, spruce and fir, with their needles; ginkgos, with their fleshy seeds; and cycads, with their large palm-like leaves and prominent cones. Around 300 million years ago a global ice age hit the planet, and the Earth became progressively drier and cooler as great bodies of ice formed at the poles and locked away precious water vapour from the atmosphere. The reduction of atmospheric moisture caused vast areas of tropical forests and swamps to shrink and dry out, and with their ability to disperse their seeds and colonise drier environments, gymnosperms soon replaced ferns as the dominant plants on the planet. In time the higher-altitude regions of the planet became regions of cold-climate peat lands and swamps, which would have resembled something similar to the boreal taiga of modern-day Siberia. In the milder lowlands, deciduous swamp forests were dominated by the seed ferns of Glossopteris and Gangamopteris, along with large clubmosses and immense horsetails. The first seeds The development of the seed saw gymnosperms become the dominant plant group between 290 and 145 million years ago. © imagebroker.net/SuperStock By the end of the Permian period the main continents of Earth’s land masses had all fused together into one supercontinent called Pangaea, and parts of the planet had become arid with little rainfall, creating extreme desert landscapes. As deserts expanded and coastlines shrank, this extreme climate shift began to push many life forms to the brink, and by 248 million years ago, 95 per cent of the plant and animal species that had evolved by this point were wiped out. This marked the largest extinction ever known, and for the next 500,000 years complex life on Earth teetered on the brink of complete extinction. The 5 per cent of life that remained was sheltered from the extreme climate, in habitats that remained temperate and moist enough. These pockets of life harboured the fundamental DNA that had evolved so far. Over the following 50 million years, as the global climate became more amenable once again, plant life would bounce back to colonise the planet. Slowly plants began again to create temperate woods, tropical forests and dry savannahs. As the Jurassic swamps and prehistoric woodlands began to spring back to life, plants continued to increase and diversify. Seeds, leaves and pollen became more specialised, and the world of plant life provided an abundance of food for the dinosaurs. Plants gave rise to fast-growing bamboo and shade-giving palms until 140 million years ago, when the plant world would be changed completely. (#ulink_b6ac49d9-3d1e-5df6-a278-fe1d5c7ee061) © Don Paulson Photography The botanical gardens and private collections of Europe’s cities were soon overflowing with an explosion of fascinating and rare flowers.’ For more than two hundred years humans have had an obsession with flowers. It has seen men give their lives in search of the most exquisite floral specimens, and caused many others to lose their minds in pursuit of the rarest. The Victorians used the term orchidelirium to describe ‘flower madness’, the botanical equivalent of ‘gold fever’ for the 1800s. This fascination with exotic flowers began with the pioneering plant hunters of the eighteenth century, who sailed to South America, Asia and Africa, travelling through unmapped territory in search of botanical wonders. These early expeditions were commissioned by wealthy collectors and botanical organisations, and they aimed to supply high society’s increasing appetite for new and exciting plants and flowers. Often spending many years abroad at a time, plant hunters risked their lives, negotiating wild animals and hostile natives, in order to discover new plant species. The finest specimens could fetch a mighty price for their scientific uses and aesthetic value. Our attraction to flowers has a deep history; evidence from a Neanderthal burial site in Iraq suggests that even 200,000 years ago our close hominid relatives were using flowers in ceremonies, laying the blooms from plants such as ragwort and grape hyacinth over the bodies of their dead. Throughout Greek myth flowers were sacred to both gods and mortals: the deep red of poppies was created from the drops of blood that fell from the slain Adonis, and the nymphs that sun-god Helios banished for their disloyalty were turned into the flowers of hellibores. In ancient Egypt roses in particular were a symbol of wealth, beauty and seduction. Guests at Emperor Nero’s great banquets were showered with their petals, and it is documented that Cleopatra used the sweet scent of rose petals to lure Mark Antony. Flowers remain a huge part of our culture today, accompanying us on the most important days of our lives – our birth, our graduation, our marriage, our death. Our gardens are now awash with bright and showy blooms from habitats from all corners of the planet – magnolias from China, geraniums from the Cape of South Africa, primulas from the Himalayas and wisteria from the Orient. In 1768 a botanist and horticulturalist named Joseph Banks set off with Captain James Cook on his first major voyage to the Pacific, where he would spend the next three years collecting, studying and cataloguing the wealth of fascinating new plant species that he found thriving on the tropical islands there. Following his return to England in 1771, Joseph Banks acted as an adviser to the Royal Botanic Gardens at Kew, a position that was later formalised. Banks gathered together a team of like-minded botanists and explorers for further expeditions. His team included the explorer and plant collector Allan Cunningham, and Scottish botanist Francis Masson, who would later become known as Kew’s first plant hunter, and who would later join Captain Cook on his second major voyage. Under Banks’ supervision the gardens at Kew fast became the world’s foremost botanical garden. Impressive proteas, cycads and bird of paradise flowers from South Africa soon filled the greenhouses, with each species transported on its voyage enclosed in a mini-greenhouse, called a ‘Wardian’ case. It was the showy blooms and delicate scented flowers which drew the most attention back home in Britain. As the plant-collecting voyages pushed deeper through the thick vegetation of tropical jungles, increasing arrays of floral shapes and colours were collected, and made their way back to the collections at Kew. Sir Joseph Banks Under his supervision, Kew’s expanding collections of exotic plants saw it become a garden of international importance. © RBG Kew Continuing Banks’ legacy, his successor William Hooker, and later his son Joseph, maintained Kew’s spirit of exploration, leading further trips to the mountains of India and Nepal. Among other species they discovered a mass of stunning new species of rhododendrons, a plant popular with gardeners across the world today. However, their plant-collecting exploits weren’t always trouble-free, and during one of their trips to the Himalayas between 1847 and 1849, Joseph and his travelling companion Archibald Campbell were arrested and imprisoned for having illegally crossed the border from Sikkim into Tibet. The two men and their botanical specimens were only released when the British government threatened to invade Sikkim. Sir Joseph Hooker Pen and ink portrait by T. Blake Wirgman, 1886. © RBG Kew As well as the public botanical collections of the time, such as Kew, obsessive private collectors also set out to acquire rare and exotic or even undiscovered flowers, which was lucrative for the financiers and explorers alike. The expeditions were often shrouded in secrecy to prevent rival groups from acquiring information as to where new species were likely to be growing, and it wasn’t uncommon for false maps and information to be circulated in order to disorientate the competition. This was the age of orchidelirium, and successful collectors could sell their prized specimens at auction for colossal sums of money. It was these privately financed trips which brought back the first orchids to Britain, from the East, and in 1852 some of them made their way into the hands of London wine merchant John Day. Bought for the equivalent sum of £3000 in today’s money, Day’s first orchid flowers marked the beginning of a lifelong obsession. His house in north London was soon transformed with the delicate white and maroon petals of Dendrobium from Southeast Asia, Odontoglossum from tropical America and Cattleya from Costa Rica. Combining his love of orchids with his keen artistic eye, Day set about documenting his increasing collection of flowers in a set of watercolours. His meticulous paintings, complete with notes on the plants’ habitat, conditions for cultivating them and their price at auction, soon caught the eye of botanists and art lovers alike, and he was given special access to the orchid house at Kew to paint its plants. Over 25 years, Day compiled over 50 sketchbooks filled with his detailed, colourful visions of these captivating plants, and these drawings can still be admired in the collections at Kew today. Plant hunting From Joseph Hooker’s Himalayan Journals, 1854. © RBG Kew The Victorian obsession with acquiring the most ornate flowers was made all the more possible by an extraordinary network of vivacious plant fanatics, who were willing to use their work in the far corners of the British Empire as an opportunity to bring back exotic species from across the globe. Colonel Robson Benson, an officer in the British forces in India, used his time on duty in Assam, Bhutan and Cambodia to collect a multitude of new species of orchid for the British horticulturist Hugh Low. Painter William Boxall, working first in Burma and later in the Philippines, collected enchanting slipper orchids, magnificent Vanda, and a number of species of the genus that today fills the shelves of nearly every garden centre, Phalaenopsis. The botanical gardens and private collections of Europe’s cities were soon overflowing with an explosion of fascinating and rare flowers, displaying an unfathomable array of shapes, sizes and colours. But as well as the aesthetic interest that drew most admirers to these flowers, their complexity and diversity provided biologists and naturalists with a wealth of material for them to study. One such naturalist was the young Charles Darwin, as well as Kew’s second Director, Joseph Hooker, who was a lifelong friend of Darwin. Darwin shared extensive correspondence with a long list of senior botanists and horticulturists at Kew, swapping notes on plants and exchanging specimens. During his time on the Beagle between 1831 and 1836 he gathered species of flowers from Argentina, Chile, Brazil and the Galapagos which he sent back to Kew for identification, and in turn Kew happily provided Darwin with plants for him to document and study at his house in Kent. Although at this point Darwin had not yet written his seminal work On the Origin of Species by Means of Natural Selection, he was already piecing together his ideas on survival and adaptation in the natural world. Perhaps more than anyone else at the time, Darwin knew that for all their beauty, the complex shapes, patterns and structures of every unique orchid flower must be a result of some advantage that they bestowed upon that species in its habitat. Darwin understood that the flowers of orchids were purely about coaxing animals to spread its sex cells. Illustrations by John Day, taken from his ‘scrapbooks’. Cattleya skinneri A species of orchid found in Costa Rica and Guatemala. © RBG Kew Illustrations by John Day, taken from his ‘scrapbooks’. Catasetum christyanum An epiphytic orchid from northern South America. © RBG Kew Illustrations by John Day, taken from his ‘scrapbooks’. Vanda coerulea A species of orchid discovered in Sikkim by Joseph Hooker in 1857. © RBG Kew Illustrations by John Day, taken from his ‘scrapbooks’. Dendrobium formosum A species of orchid first discovered in northeast India. © RBG Kew Darwin’s instincts as a naturalist stemmed from his love of collecting, and his belief that in order to understand any aspect of the natural world, one must acquire, and carefully examine every facet of it. He once wrote, ‘By the time I went to school my taste for natural history, and more especially for collecting, was well developed. The passion for collecting, which leads a man to be a systematic naturalist, a virtuoso or a miser, was very strong in me, and was clearly innate, as none of my brothers or sisters ever had this taste’. In his quest to make sense of the elaborate flowers of the orchid family Darwin began amassing his own collection of these rare plants, which he held in his glass conservatory at Down House. Countless orchids from Malaysia, the Philippines and Central America made their way via Kew to his house, together with the British species which grew in abundance nearby. But the nature of the most extreme orchid flowers did not fit well with his theory of evolution. In one letter that Darwin wrote in 1861 to John Lindley, who worked as one of Kew’s taxonomists at the time, he describes his utter fascination with the complexity of orchids, discussing one genus in particular called Catasetum: ‘I have been extremely much interested with Catasetum, and indeed with many exotic orchids, which I have been looking at in aid of an opusculus, on the fertilisation of British Orchids. I very much fear that in publishing I am doing a rash act; but Orchids have interested me more than almost anything in my life. Your work shows that you are carefully understanding this feeling.’ Deception The labellum of this mirror bee-orchid has evolved to mimic the shape and shine of an iridescent bee. © RBG Kew Darwin studied the lives of orchids and dissected them, looking at the multitude of ways in which the plants guided specific bees or moths to their flowers to interact with their reproductive structures, and the mechanisms they exhibited to achieve pollination. He was searching for an explanation for all aspects of each flower’s behaviour, and a justifiable origin for each. But for many of his adversaries, what Darwin was trying to achieve was considered impossible, and even his good friend Thomas Huxley famously stated, ‘who has ever dreamed of finding a utilitarian purpose in the forms and colours of flowers?’ Darwin made good headway in unravelling the sex lives of orchids, and he made detailed studies of the ways in which they lured pollinators and released their pollen. But what had him most stumped was that, more so than any other family of flowers, orchids exhibit extreme pickiness in whom and what they allow to spread their pollen. On the subject of this he wrote: ‘Why do orchids have so many perfect contrivances for their fertilisation? I am sure that many other plants offer analogous adaptations of high perfection; but it seems that they are really more numerous and perfect with the Orchideae than with most other plants.’ What seemed counterintuitive to Darwin was that for all their elegance, the pollination methods employed by orchids seemed terribly inefficient. Darwin’s trouble with trying to explain the nature of the sex life of orchids becomes all the more apparent as soon as you begin to unfold the highly specialised ways which we now know different species achieve pollination. In the mirror bee-orchid (Ophrys speculum), found in southern and western Europe, as well as Lebanon, Turkey and North Africa, the lip of the flower looks nearly identical to an iridescent bee. This was first suggested to be a ploy to prevent grazing animals from munching on it, but we now know that it in fact releases a chemical that mimics the pheromones of a female bee, as a trick to get males to ‘mate’ with it. By rubbing its body on the flower in an attempt to copulate, the male bee will rub itself up against the plant’s sticky bundles of pollen, called pollinia, which adhere to its body, before it flies away and attempts to mate with another bee-orchid. Another extreme behaviour has evolved in the orchids of the genus Oncidium from Ecuador, which have petals that look like the insect competitor of the Centris bee. The bee attempts to chase this ‘enemy’ away from its territory, and in doing so it strikes the flower, showering itself in the plant’s sticky bundles of pollen. The slipper orchids of Asia and South America have a hinged lip which forces insects to brush past the sticky pollen before leaving, and there is even an underground species of orchid from Australia called Rhizanthella slateri which relies on ants to move its pollen. Other orchids emit a smell of rotting flesh to attract meat-loving flies to pollinate them, while some have been found to smell like chocolate. Perhaps the most intriguing pollination syndrome is that of Catasetum, which is so complex it seemed to contradict Darwin’s very theory of evolution. Unusually for orchids, some Catasetum plants are male and others are female. The male produces a scent that attracts just one species of euglossine bee. Lured by its sweet smell, the bee lands on the lip of the orchid and thrusts its head into the flower, touching a hair-trigger. This activates a mechanism that fires out a tiny bundle, which then sticks onto the bee’s back. This extraordinary projectile is in fact a bundle of pollen grains called a pollinium, which has a little cap on it, and after a minute or so the cap falls off to reveal a little horseshoe-shaped bundle of pollen grains. A group of researchers in the USA recently found that the pollinium is ejected with an acceleration rate of over ten times that of a striking pit viper. Having been struck by this pollen, the bee flies away and is attracted to another rather different-looking flower, which is the female. Once again, lured by the scent, it sticks its head into the female flower – and the little bundle of pollen attached to its back, like a key, fits into a small aperture on the roof of the flower, like a lock, pulling off the pollen as the bee makes its departure. Pollination has been achieved. The Catasetum conundrum The structures of these orchids interested Darwin immensely on account of their incredible complexity. © Will Benson Darwin’s obsession with Catasetum in particular caused him to dedicate a great deal of time to studying the flower’s mechanism, in an attempt to make sense of how it could have arrived at such a precise and specific system. His tireless persistence paid off, and in a letter to his publisher John Murray in 1861 he wrote, ‘I have had the hardest day’s work at Catasetum and the buds of Mormodes, and believe I understand at last, the mechanism of movements and functions. Catasetum is a beautiful case of slight modification of structure leading to new functions.’ Having unravelled the complexities of exploding pollen bundles, it wasn’t long before Darwin’s next botanical mystery would land on his desk, quite literally. In 1862 he received a package from renowned horticulturist James Bateman, a striking orchid with a flower composed of large star-shaped white petals from the island of Madagascar, named Angraecum sesquipedale. Darwin set about detailing the ornate nature of his latest specimen and was struck by the long tubes, called spurs, in which the plant’s nectar was contained. Its delicate spurs were over 30 centimetres long, hanging down beneath the flower like white tails, with the nectar contained at their tip. Having never seen anything quite like this before, he wrote to his esteemed friend Joseph Hooker to explain these foot-long, whip-like nectaries, exclaiming, ‘Good Heavens what insect can suck it!’ Later that year Darwin went on to publish a book on the reproduction of orchids, in which he theorised that in order for the Madagascan orchid to be pollinated, an insect, most probably a moth, must exist on the island of Madagascar with a tongue at least 30 centimetres long which can reach the nectar at the end of the spurs. His suggestion seemed ludicrous to many of his peers, but a paper written by fellow evolutionary theorist Alfred Russel Wallace a few years later sought to back up Darwin’s notion, by highlighting that a large hawk moth had been discovered in Africa which had a tongue almost 20 centimetres long, called Xanthopan morgani. Wallace predicted that if such a moth existed in Africa, then surely a moth with a 30-centimetre-tongue could live in the forests of Madagascar. Unfortunately Darwin was never able to see his prediction come true, but in 1903 a population of hawk moths with the necessary tongues were found on Madagascar. The team who discovered it then aptly named it Xanthopan morganii praedicta – the predicted subspecies of X. morgani. Mystery of the moth The relationship between hawk moth and Angraecum sesquipedale is one of the greatest examples of plant and animal co-evolution. © Minden Pictures/SuperStock But for all the sense that Darwin was able to make of the lives of flowering plants and how they disperse their pollen, there was one fundamental aspect of their world that he was never truly able to fathom. He couldn’t understand how flowering plants had come to exist in such diversity in such a short period of geological history. For almost half a billion years, plants had existed without flowers, and then in a few million years they appear in the fossil record as the dominant form of plant life. In a letter sent to his friend Joseph Hooker in 1879, Darwin remarked on his puzzlement on the sudden radiation of flowering plants, stating, ‘the rapid development, as far as we can judge, of all higher plants within recent geological time is an abominable mystery.’ It went against his very theory of ‘natural selection’ that he had outlined for the rest of life on Earth. Darwin’s work, which observed the processes by which all species struggle for survival and compete to reproduce in order to pass on their genes, helped build his theory of evolution – the process by which a species over many generations acquires novel and advantageous traits. From his acute observations of the birds, insects and reptiles of the Galapagos Islands, together with the fossils of extinct animals that he gathered during his voyage on the Beagle, Darwin discovered that the change inferred to organisms over time was a slow and gradual process. In his subsequent book On the Origin of Species by Means of Natural Selection he states that ‘natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by short and sure, though slow steps.’ However, for all the diversity that Darwin could see in the modern world of flowering plants, the fossil record revealed no trace of the expected slow and gradual transition of non-flowering plants to those with flowers. For Darwin, the seemingly sudden appearance of flowers contradicted the very rules of evolution. In the fossils of the Carboniferous period horsetails and early seed plants dominated the land. In the fossils from the time of the dinosaurs cycads, ginkgos and ferns were dominant. And then suddenly in the fossils of the Cretaceous period, 130 million years ago, an explosion of flowering life appears and takes over the land. In this short period all of the major groups of flowers that we see today emerged. Not only did each species of flower look different, but they had developed a multitude of reproductive styles – from the relatively straightforward mechanism of those flowers which released their pollen on the wind, to those possessing nectar-filled organs for the more complex task of luring insect pollinators. This sudden spurt of evolution not only had Darwin stumped but has continued to boggle botanists for the last 120 years. Darwin knew that the fossil record was not a wholly complete snapshot of life through time, and he used this to try and explain the sudden burst of flowering plants on Earth. Due to the fact that plants do not have hardened body parts like the easily fossilised internal and external skeletons of many animals, there is a chance that the intermediate stages of the first flowers may simply have decomposed and been lost when they died. In his correspondence with Hooker, Darwin suggested that flowering plants had perhaps evolved slowly and that the fossils were yet to be found. Another suggestion was that a rapid increase in flower-frequenting insects in the Cambrian may have spurred their evolution on. In the animal world it is possible to see many of the intermediate steps that have given rise to certain animals. The embryos of snakes, dolphins and whales all sprout the buds of vestigial legs when they are embryos, echoing their evolutionary past, which then shrink and disappear before they are born. However, plants do not retain these evolutionary features in the same way that animals do, and it is far harder to trace the steps by which flowers came to be by studying their more primitive relatives. What makes things harder still is that the flowers of even moderately primitive groups of flowering plants are so different from their assumed extinct relatives among seed plants, that it is incredibly difficult to reconstruct a plausible evolutionary history for them. However, since Darwin’s day new fossil finds and our considerable advances in genetics have helped us begin to unravel the origins of flowering plants. In the mid-1980s an international collaboration of over 40 scientists from around the world, coordinated byKew geneticist Professor Mark Chase, embarked on a mammoth project. Over a number of years the team meticulously extracted the same type of gene from over 500 different types of flowering plant, and by the early 1990s they had gathered enough information to begin to compare them. By looking at the plants which shared the most similarities for this type of shared gene, Professor Chase and his team were able to work out which groups of flowers were more closely related, and by pinpointing those in which the gene had considerable differences they could assume they had evolved separately. The findings, published in 1993, allowed them to piece together an accurate tree of life for flowering plants. Fifteen years later a team of researchers at the University of Florida built on this tree of life to create a more complex timeline for the emergance of different types of flowers. By looking at the genes of a number of living plants that could be linked to their fossil ancestors from known dates in prehistory, the team were able to work out the rate at which certain genes changed over time. The results from these calculations gave them a ticking genetic clock which could then be used to date the origins of the first flowering plants. The major revelation from their work was that previous estimates for the first flowers had been inaccurate by around 10 million years, and that the first blooms were in fact evolving as far back as 140 million years ago. But the Florida team’s findings still seemed to indicate that flowering plants did indeed rapidly radiate in as little as just five million years. For years the debating and theorising continued as to how plants could have seemingly cheated evolution, to quickly rise from relative obscurity into a wealth of developed flowering structures during the Cretaceous period. Then scientists believed they had found an explanation, in the form of a happy coincidence, a genetic mishap discovered in some plants known as polyploidy. It has been known that when the male and female haploid sex cells of both plants and animals combine during reproduction to create the next diploid generation, some sections of genetic information from the parents can become duplicated in the new generation. In the case of humans, the accidental insertion of any additional genetic information can be extremely damaging for the child’s health; even the duplication of just one of our 46 chromosomes will cause Down’s syndrome, and the duplication of two or more chromosomes would be fatal. However, flowering plants with their comparatively simple body parts have been found to be able to live healthily with accidental duplications of genetic material, even in extreme cases where the whole of a plant’s genome (i.e. every one of its chromosomes) becomes duplicated. Not only can a plant species tolerate these polyploidy events, but it appears that they can actually thrive on them. It had long been considered that flowering plants’ ability to duplicate large parts of their genetic information could have been a contributing factor that allowed them to increase in diversity at an abnormal rate, and a study carried out in the 1970s calculated that between 30 per cent and 80 per cent of all flowering plants have undergone a multiplication of parts or the whole of their genome at some point in their evolutionary history. Plants that have undergone polyploidy are typically more vigorous. By tracing through the lineage of many different groups of flowering plants, scientists have now found proof that a number of polyploidy events does indeed explain the fast rate at which particular types of flowers radiated, such as the prolific grasses, the nightshades, the pea family and the mustard flowers. But what of all the other flowering families that make up the 400,000 or so species on Earth today? Nuphar lutea These yellow water-lilies are among the oldest living relatives of the first flowering plants. © Chris Cheadle Up until very recently speculation still remained as to what exactly could explain the apparently sudden explosion of all flowering life 140 million years ago. Then in 2011, at a conference of the International Botanical Congress in Melbourne, a palaeontologist from the Swedish Museum of Natural History called Else Marie Friis revealed findings which outlined a previously unseen trove of exquisitely preserved primitive flowers from the charcoal fields of Catefica, Torres Vedras and Famalicao in Portugal, dating back to the Early Cretaceous. These amazing fossils, many of which were preserved in three dimensions, gave the first glimpse of what early flowers looked like as they began to evolve, and in breathtaking detail they showed the first stages of flowering life on Earth. Some possessed clusters of small flowers grouped together to form one larger inflorescence, much like a modern-day sunflower, while other plants had small single flowers no more than 2 millimetres across. Most seemed to have few floral parts, and many even lacked petals and the protective outer sepals which are characteristic of most modern flowers. Numerous seeds and pollen were also found in the fossils, and the high number of fruits possessing fleshy coats suggests that animals had a key role in dispersing the seeds of these plants. Friis’s fossils seemed to reveal what flowering plants looked like some 30 million years before the fossil evidence of Darwin’s day, at a point when they were first acquiring the features which would ultimately lead to the flowers we see today. While historic polyploidy events undoubtedly gave ancient flowering plants occasional moments of accelerated radiation in their shape and form, these fossils revealed that the overall rise of flowering plants was far more gradual than Darwin had thought, and that, like all life on Earth, they had evolved their structures through a process of gradual change. Buzz-pollination The stamens of Gustavia longifolia only release their pollen when buzzed by the wings of a bee. © RBG Kew From the time when these first flowers became immortalised in the coals of the Early Cretaceous, the angiosperms – as all flowering plants are collectively known today – have since diversified into a huge range of specialised species, each one with its own way of encouraging its pollinators to disperse its pollen. Fast-growing, compared to the ancient cycads and conifers, and able to tolerate fluctuating climates, flowering plants soon became the most species-rich plant group on Earth. Relatives of the earliest flowers to evolve can still be found today, the oldest of which is a plant from the cloud forests of New Caledonia called Amborella, and Nuphar, a water-lily. The first pollinators of early angiosperms are thought to have been flying insects like scorpion flies that, having been partial to the nectar of seed ferns, would have been easily lured by the blooms of the first flowers that emerged. Angiosperms fast began to optimise their blooms to make them more enticing to particular kinds of pollinators. Over time flowers became bigger, brighter and more scented, and as flowers began to evolve to favour the tastes and temptations of certain animals, the pollinators in turn began to evolve to maximise their ability to drink nectar or eat the nutritious starchy pollen of particular flowers. One particularly ingenious example of a flowering plant which has maximised its pollen-spreading success is the flowering tree Gustavia longifolia, from the western Amazonian forests. A team of tropical horticulturalists who studied the plant at the Royal Botanic Gardens at Kew found that its fleshy, deep-purple flowers have acquired a very clever method to ensure that its pollen is only taken by the particular kinds of bee that are likely to spread that pollen to other G. longifolia flowers. A species of night-flying bee climbs in among the stamens of the flower to feed from the nectar inside, and as the bee drinks from the sugary fluid the frequency of its buzz causes the flower to shake violently, at a force calculated to be as much as 30 times the pull of Earth’s gravity. These violent vibrations shake the flower’s anthers, and in the process its sticky yellow pollen is released and showered over the bee’s back. This clever mechanism, called buzz-pollination, occurs in a number of other unrelated flowers from all over the world. The flowers of the tomato plant, for instance, release their pollen for only a handful of species of bee. Farmers who grow acres of the plants have tried to trick the flowers into releasing their pollen by using vibrating tuning forks or buzzing electric toothbrushes – but nothing provides as good a pollination service as the bumble bee that has evolved alongside the tomato plant for millions of years. Flowering plants are one of the most successful life forms on the planet, and they have come to occupy almost every known habitat on Earth. But while bees are indeed the most prolific pollinators, there are many other species which help move pollen from one flower to another. Some of the plants alive today, whose ancestors evolved shortly after the primitive flowers of the Early Cretaceous, such as the water-lilies and the magnolias, evolved a wealth of tactics to persuade flies and ancient beetles to feed and transfer their pollen. As well as their heady scents and enticing blooms of electric blue, loud pinks and mesmerising yellows which make them irresistible to insects, they are also able to produce heat. This ability, known as thermogenesis, provides a warm landing place with a ready supply of nutritious pollen, and a flower is therefore an easy choice for any insect looking for an inviting place to visit. Flowering giant Structures at the base of the titan arum’s spathe produce the pollen which is picked up by visiting insects. © Rob Hollingworth From the bird world, hummingbirds are prolific pollinators of bright red jungle plants such as honeysuckle, using their long beaks to gather nectar from the trumpet-shaped flowers. Moths pollinate some of the more ghostly flowers, such as those of the night-blooming cacti Echinopsis and Selenicereus, and butterflies are responsible for pollinating many thousands of species of pink or lavender-coloured tropical flowers such as the Asian buddleja or the American passion-vine. Snails and slugs smear pollen from plant to plant as they move through vegetation, and mosquitoes pollinate some species of orchids. Mammals too, both on the ground and in the air, transfer pollen for many hundreds of different flowers. Even lizards on the island of Mauritius have been found to transport the sticky pollen of particular plants with tough flowers, as they forage for fruits. Since the arrival of flowers, the animal world has been inextricably linked with the plant world, and for the past 140 million years they have evolved together. In most cases it is a mutually beneficial relationship, in that the animal gets a meal, and the plant spreads its DNA. The plant world’s ability to harness the hungry nature of animals and get them to carry their pollen is the greatest trait that plants have acquired, and as long as there are animals waiting to get a meal, flowering plants will remain the dominant and most fascinating organisms on the Earth. Mankind’s obsession with flowers has not waned since Victorian times, and although orchids are still the chosen obsession of many, thousands of different flowering species are now cultivated and admired in gardens and conservatories around the world. Our continued love of flowers is personified today in the beds of the Royal Botanic Gardens at Kew, now a UNESCO World Heritage Site, where flowering plants from all corners of the globe attract over two million people every year to marvel at the variety of the plant world. The Palm House is home to exotic tropical plants from jungles all over the world, and the grand structure of the Temperate House contains thousands of temperate and cool-zone plants from Asia, Australia, Africa and the Americas. But it is in one of the more recent additions to Kew’s landscape that the gardens’ biggest draw can be found. In the Princess of Wales Conservatory, the most technologically advanced greenhouse in the world, there is a plant that since its discovery in Asia in 1876 has not ceased to fascinate all who see it: the titan arum (Amorphophallus titanum). The plant was first discovered by an Italian botanist called Odoardo Beccari, who stumbled across it during his expeditions in the tropics of Sumatra. He packaged up some of its seeds and hastily sent them back to Europe, and when a handful of these germinated a young plant eventually made its way to Kew. For over 10 years the plant grew in size at Kew, putting out mighty leaves, the size of a small tree, until in 1889 it finally produced its first flower. Amazingly, the single triffid-like bloom which emerged from this almighty plant was as large as its 2-metre-tall leaves. It seemed clear that the titan arum must surely be the largest flower in the world. However, on closer inspection it was found that its great totem-pole-like structure was actually made up of many thousands of minuscule flowers, making it by definition an inflorescence and not a single flower. Instead the accolade of the largest single flower is held by the metre-wide parasitic species Rafflesia arnoldii, which also grows in the tropical forests of Sumatra, and across Southeast Asia. Nonetheless, the titan arum is a botanical giant, and rising to 3 metres tall in its mightiest specimens, its bloom towers over any human. Its flower body consists of a frilled purple collar around a tall speckled-green and cream-coloured flower-bearing spike, which is made of separate male and female flowers. Only staying open for a couple of days, the titan arum has to attract as many of its pollinators as quickly as it can, and it does this by emitting a foul and fetid stench, described as resembling rotting flesh, sour dairy and burnt sugar, which is produced by sulphur-containing compounds on its spike. The stench of the recently opened flower is in fact so vile that the artist who came to draw the first specimen that came into flower at Kew was made ill after inhaling it for too long. Shortly after opening, the base of the flowering spike begins to generate heat of around 36°C, which creates a convection current to help waft its rancid smell through the night air. By burning reserves of stored carbohydrates, the plant produces heat in waves over a few hours, and the resulting pulse of scent that is emitted acts to punch through the layer of cooler air that forms below the forest canopy. Once through this layer, the foul scent is able to travel great distances through the forest and reach the olfactory organs of its pollinators. Any carrion-beetles or flesh-flies that catch a whiff of its odour will then hungrily fly to the flower expecting to find a meal of decaying meat, and on arrival will bump into the flower spike. Cunningly, the male and female parts of the titan arum open on separate nights to prevent the flower from self-pollinating, so it has female parts at the bottom which open on the first night, and male parts on the top which open later. As flies search the flower for the source of the rancid smell they become caught in the deep cone of the flower’s collar, and in order to escape they must crawl up the flower spike, getting coated in pollen as they go. They then fly to another open flower and, starting at the bottom again, crawl upwards – and in doing so cross-pollinate the flowers. Since the 1800s titan arum has flowered numerous times at botanical gardens across the globe where it is now showcased, perhaps most notably at Kew in 1926, when police were called in to control the huge crowds that had gathered to see and smell the much-talked-about spectacle. Titan arum plants are now grown by botanical institutions and private collectors all around the world, but the occasions when their blooms emerge still make the headlines. Door-to-door transfer Flowers provide an elegant door-to-door pollen delivery system for plants. © Minden Pictures/SuperStock The dizzying diversity of flowering plants today is truly staggering, and we continue to discover further members of these incredible plants, such as orchids which only flower under the cover of darkness and palm trees which flower themselves to death, flowers which imitate an incredible array of insects, and even flowers which mimic other flowers. In their long history, spanning over 400 million years, plants have developed many amazing strategies to better survive and reproduce, and the evolution of the flower is surely one of their greatest. Not only has it allowed angiosperms to outnumber their fern and conifer ancestors by 20 to 1, it has also helped forge the relationship between humans and plants. Many of the plants that today support the human population, such as the grasses that provide cereals and sugar, the many fruits and vegetables we eat, as well as cotton, coffee and chocolate, and trees that provide building materials, are the result of the evolutionary success of these flowering plants. (#ulink_3ebaecb1-3d84-501e-ba51-7469420a17e3) © Will Benson ‘Throughout history humans have looked for patterns in nature.’ Long before humans built the first houses out of mud, straw, wood and vines, plants employed organic material to create a multitude of structures, each one more advanced than the last. Plants use the powers of speed and size to push out the competition and overwhelm their adversaries. On the exposed slopes of wind-lashed mountainsides, plants use the powers of endurance and timing to survive the bitter winters. And in the bleakest deserts, plants use extreme structures to protect their bodies and employ chemical tactics to protect them from enemies. The species that we can see in the world around us today bear the scars of their evolutionary history. They are the ones who passed on their genes more successfully. Every thorn on their stems, every ridge on their leaves and every berry on their branches is the accumulated result of millions of years of evolution. For every extreme of shape and structure that appears in the plant world, there is a story of adaptation and survival, to a particular climate or lifestyle that can explain them. Every major habitat on Earth provides conditions favourable to a particular set of plants, which is why you will never find a drought-tolerant saguaro cactus growing in the middle of a moist rainforest, and you will never find a tropical mahogany tree setting down its roots in a desert. Their 450-million-year evolutionary journey has given each species of terrestrial plant a unique set of tools for survival in its specific environment, be it in the forests, the grasslands or the deserts. In each different habitat the temperature, the amount of water, the availability of light and the terrain itself determine which groups of plants can thrive there. In turn the plants and animals which already live in a habitat can limit or facilitate the addition of other species. In this way, complex ecosystems are built around an intricate network of plant, animal and fungal life, where each species is reliant on those species below it in the food chain, and in turn gives life to those above it. Forested habitats provide an environment most similar to those in which plant life first emerged during the Devonian explosion, and it is in these habitats that we see the greatest diversity of life today. Rainforests alone contain more than 50 per cent of the world’s plant and animal species, and collectively, tropical rainforests, boreal forests and temperate forests make up 30 per cent of the Earth’s landmass. This makes them the most important habitat type in terms of the carbon-capturing and oxygenating services that they provide our biosphere with. Historically forests have been a lifeline, harbouring the survivors of the ice age that hit Earth at the end of the Cambrian. Plants in this habitat managed to survive, while those elsewhere perished in the cold, dry climate. These forests now make up the oldest continuously growing habitats on Earth, at around 135 million years old. In comparison, the tropical expanse of the Amazon is a relative infant at only 40 million years old. The ancient forests that sheltered the DNA of plant life on this planet can still be visited today, in places like the Daintree Rainforest on the northeastern coast of Queensland in Australia. Ancient refugia Australia’s Daintree Rainforest is the oldest continually growing habitat on Earth. © imagebroker.net/SuperStock In modern-day forests trees dominate the landscape, and their layers of leaves and branches create niches where many other forms of life can grow. The tallest trees benefit from broad canopies which can stretch above all other plants to absorb the most light, while plants living below have to adapt to make best use of the limited light that filters through to the lower layers. However, the trees at the top of the canopy must also endure the hottest temperatures and lowest humidity. Some of the largest forest trees are the largest plants on the planet: for example the giant sequoia, Sequoiadendron giganteum, which grows in the evergreen forests of central California. The largest can reach over 80 metres high, and contain enough wood to build over 40 small houses. One tree can support as many as a hundred other species of plant and animal. Woody vines called lianas use the tall trunks of trees to creep up towards the light at the top of the canopy. As they rely completely on the support of other vegetation they invest little energy in structural support, and as a result they put all of their resources into rapid growth and leaf production. There are over 2500 species of lianas which grow in the tropical forests of Africa, Asia and the Americas. As they grow, these tiny vines may remain thin and cling to the sides of trees, or may ultimately become colossally thick stems which appear as tough as tree trunks themselves. In some forests lianas have been found to make up over 40 per cent of the total leaves in the canopy. Climbing vines The vascular system of lianas has evolved to transport water hundreds of metres through the canopy. © RBG Kew Lianas take up water and nutrients for the plant from the forest floor, but in order for them to reach up to the top of the canopy they must be able to transport their nutrients through their elongated stems – as far as 900 metres in the most extreme examples. While the evolution of lianas has resulted in one of the most advanced water transport systems of any plant, there is a group of specialised plants that have found a way of living high up in the architecture of the forest trees without the need for long roots. These plants are known as epiphytes, or air plants, as they do not have roots which grow into the soil, but instead have short roots which can absorb nutrients and water from organic matter that accumulates on branches high up in the forest canopy. As these plants must absorb all of their water from the air, they can only thrive in very humid environments, and in the high-altitude montane forests of the tropics trees can become covered in epiphytic plants. Epiphytes include the mosses and lichens of temperate forests, but also a multitude of more complex plants such as orchids, ferns and some tropical cacti. However, there is one group of plants that stands out as the true masters of arboreal life, the tank bromeliads. Relatives of the pineapple, tank bromeliads live attached to the branches of trees in the rainforests of South America, where they display an amazing variety of shape and colour, from broad, green fleshy-leaved plants, to small, delicate purple and red structures. Their broad leaves are arranged into a basket-shaped rosette which acts to channel the rainwater that trickles through the forest canopy into a central reservoir, and the bases of these leaves are packed so tightly together that they are able to create a watertight tank in which the water can gather. In the largest species of tank bromeliad as much as 50 litres of water can be held between the plant’s leaves, and a study conducted in Puerto Rico found that in just a 1-hectare area of rainforest as much as 50,000 litres of water can be stored, which would fill a small swimming pool. Tank bromeliads These plants provide arborial homes for a whole host of animal life. © RBG Kew The little elevated lakes that gather in the bases of bromeliads make perfect homes for a handful of species looking to escape the perils of the forest floor. Insects such as mosquitoes lay their eggs in the pools of water, and flatworms find shelter among the leaves. With this abundance of insect life comes a menagerie of larger animals that visit the pools to munch on the invertebrate feast. Inch-long salamanders come to feast in the relative safety of the plant, and in some bromeliads in Jamaica small crabs have been found to dwell, territorially defending their plant against lizards and millipedes. Tiny poison dart frogs are perhaps some of the best-known lodgers, with some species spending their entire lives, from tadpole to adult, inside the seclusion of the bromeliad’s tank. A recent count of the different animals that live in the bromeliads of Ecuador found an astonishing 300 different animal species that made these plants their home. However, the bromeliad does not just provide a home for these animals to be of service, rather it accommodates them so they can provide it with food. The droppings of the frogs and salamanders, which contain the digested bodies of the insects that gather in the pool, accumulate in the water and can be absorbed by the plant as a vital source of nitrogen-rich food. Back on the forest floor plants grow in a more conventional way, rooted to the ground. Although these plants only receive around 2 per cent of the glorious sunlight available at the top of the canopy, they have the advantage of growing directly on the nutritious layer of organic material created by an unseen army of bacteria and fungi. Species here are able to spread across the forest floor in a dense carpet of growth, employing a number of adaptations which allow them to thrive in the forest understorey. One such strategy is seen in the stripy-leafed Tradescantia zebrina from the forests of southern Mexico, which has purple undersides to its leaves created by a pigment called anthocyanin. As light passes through the green photosynthetic tops of the leaves the purple cells underneath act like a mirror and bounce the light back up to ensure that the maximum energy is absorbed by the plant’s chlorophyll. Other plants use size to their advantage to capture as much light as is physically available. One of the most successful plants at using this strategy is the giant taro plant, Alocasia robusta, which has the largest undivided surface area of any leaf on the planet, growing to over 3 metres in length and over 2 metres in width. Its huge glossy leaves thrive in the understorey of the tropical forests of Asia, where they fan outwards in order to gather light throughout as much of the day as possible. Another more subtle mechanism used by plants to ensure they can gather as much light as possible is found in a small purple-leafed shade-dwelling species called Oxalis oregana, from the redwood forests of western USA. At the top of its short 15 cm stems it has triplets of heart-shaped green leaves which are able to move in order to track the sparse sunlight as it shines through the canopy above. However, as Oxalis has adapted to photosynthesise in such low levels of light, strong sunlight can actually be damaging to its cells. Consequently, should a beam of sunlight break through the canopy directly onto its leaves, in just 6 seconds the plant can tilt its leaves to a vertical angle and escape the light. Another group of prolific plants that thrive in forests are the palms, notable for their economic importance to us. Found growing both as tall trees with mighty crowns poking through the forest canopy and in short, spiky clumps at ground level, palms can live in habitats ranging from the desert islands of the hot tropics to the milder Mediterranean climes of the subtropics. They are instantly recognisable by their distinct leaves. Resembling thick, green feathers and broad, fan-like paddles, their leaves give them a surface for absorbing energy from the sun, and their deep ridges channel rainwater away from their surfaces. Various animals also rely on palms for food and shelter: birds such as palm-nut vultures and macaws flock to the plants to eat their fleshy fruits, and small, ground-dwelling mammals like the Asian palm civet (Paradoxurus hermaphroditus) forage for fruits about their bases. Madagascar has the highest diversity of palms anywhere on the planet, and because of the island’s lack of herbivores the leaves of its palms lack the chemical defences and spines found elsewhere. Some of the island’s palms have leaves that extend for no more than a few centimetres, but in the extreme example of the raffia palm (Raphia farinifera), its leaves can hang down from the crown a massive 24 metres – the longest known leaf of any plant, roughly the height of a seven-storey building. The largest seed of any known plant comes from a palm too, called the coco-de-mer or double coconut, and the largest known inflorescence comes from a species of palm called Corypha umbraculifera. The eighteenth-century Swedish naturalist and father of taxonomy, Carl Linnaeus, was so taken by palms that he labelled them as Principes, the order of the Princes. Leaf architecture Leaves are the food-making factories of green plants, and come in a variety of shapes and sizes. © Will Benson Palms today provide humans with an array of useful materials and foods, and after grasses and legumes they are the most economically important plants on the planet. Nearly every part of the palm plant can be used for food: the sap is commonly boiled to create a sugary food called jaggery, and the oils from the flower are tapped to make a fresh drink or fermented and distilled to create a number of potent alcoholic beverages, such as the east Asian liquor arak. The sweet tips of the fresh leaf growth make a sweet salad, and the starch from the fibres of the trunk can be harvested to make a nutritious food called sago. As well as their multitude of edible uses, palms provide a variety of practical materials, both locally to where they are grown and across the world. Their wood is used to make buildings and furniture, as well as ropes, clothing and fibres, and the oil from their fruits can be turned into waxes, fuel and cheap cooking oil. Sadly, the oil produced from palms is in such high demand that great swathes of diverse tropical rainforest are being felled in Southeast Asia to be replaced with vast seas of oil-palm plantations. Mighty bamboo This member of the grass family is the strongest plant on the planet. © RBG Kew Less biologically rich than their tropical counterparts, the temperate forests are characterised by leafy deciduous vegetation and conifer-covered mountains. Unlike the tropics, which often have both wet and dry seasons, temperate regions have four distinct seasons of varying warmth and precipitation, and through this cycle some of nature’s most striking landscapes are created. In the mountain region of south-central China some of the most dramatic habitats of temperate vegetation can be found, nourished by the waters of the five great rivers of Asia: the Mekong, Irrawaddy, Yellow, Yangtze and Salween. These are the most biodiverse temperate habitats, and among the mountain woodlands giant pandas feed on one of the most important plants of these forests – bamboo. This mighty plant makes up a key part of the understorey of the temperate broadleaf deciduous forests. Bamboo is a member of the grass family Poaceae. In fact, bamboo is the largest grass in the world. Separating it from all the other grasses is its tough, woody stem. This gives the strongest shoots the strength of mild steel (able to withstand around 52,000 pounds per square inch, a pressure that could crush stone), making it the strongest plant on Earth. Shooting upwards like an extending telescope, each new section of the plant extends from the centre of the old sections, and the fastest species are able to advance towards the light at a staggering rate of over 5 centimetres per hour. This amazing growing capability makes bamboo a crucial plant in its forest habitat, acting as an unrivalled soil erosion control agent. Bamboo is particularly successful at re-colonising areas of land that have previously been cleared for agriculture or cattle grazing, and the re-greening of an area of land by bamboo can help return structure and life to the forest environment. Like palms, bamboo provides us with a vast variety of building materials and foods. Bamboo-related industries are estimated to provide a livelihood for around 1.5 billion people worldwide, making it a plant of great economic importance. In Asia it is used to create high rigs of scaffolding, some over 100 metres tall, and in Central America an area of farmed bamboo forest of just 60 hectares can provide enough material to build around a thousand small houses. But it is actually the less impressive relatives of the grass family that provide humans with an even greater service. These are the grasses that give us wheat, corn, rice and maize, and that feed the animals which give us meat, leather and wool. They are the most economically important plants on the planet, and their exploitation has shaped the face of the world as we know it. Wild grasslands Africa’s vast protected national parks provide a snapshot of how Earth’s ancient plains would have looked. © Flirt/SuperStock In their wild form grasses grow primarily in the semi-arid grasslands and savannahs that make up about 20 per cent of the Earth’s surface. In the past, thousands of grazing animals such as elk, wild horse and saiga antelope inhabited these vast grasslands. Today these wild habitats are much quieter. Hunters who roamed the grasslands some 20,000 years ago caused the mega-herbivores that once dominated the landscape to disappear. Today it is primarily livestock that inhabit these landscapes. Protected habitats such as Yellowstone National Park in the USA and the Masai Mara National Reserve in Kenya now form some of the few grassland areas where one can get a glimpse of how our grassland ecosystems would have looked without human interference. Plants that grow in grasslands are highly resilient to drought, and are often able to withstand months without rainfall. Short hairy grasses such as june grass (Koeleria cristata), which grows in North America’s dry prairies, have shallow roots that sit just under the surface of the ground, so that as rainwater soaks into the ground it can be absorbed by the plant. Conversely, taller grasses like elephant grass (Pennisetum purpureum), which grows to around 5 metres and, as its name suggests, is a favoured food of Africa’s largest herbivore, have hair-like, branching roots reaching up to 6 metres in length, enabling them to reach sources of water held deeper in the soil. As a means of saving water, many grasses also have long, narrow leaves which lose far less water in the drying sun than larger leaves would. Конец ознакомительного фрагмента. Текст предоставлен ООО «ЛитРес». Прочитайте эту книгу целиком, купив полную легальную версию (https://www.litres.ru/pages/biblio_book/?art=39753801&lfrom=390579938) на ЛитРес. Безопасно оплатить книгу можно банковской картой Visa, MasterCard, Maestro, со счета мобильного телефона, с платежного терминала, в салоне МТС или Связной, через PayPal, WebMoney, Яндекс.Деньги, QIWI Кошелек, бонусными картами или другим удобным Вам способом.